1. Field of the Invention
The present invention relates to a Group-III nitride crystal substrate that is of low dislocation density and is inexpensive to manufacture, to a method of manufacturing such a substrate, and to Group-III nitride semiconductor devices that incorporate the Group-III nitride crystal substrate.
2. Description of the Background Art
To serve as the substrates for semiconductor electronic and optoelectronic devices including semiconductor lasers and LEDs (light-emitting diodes), Group-III nitride crystal substrates of large size and low dislocation density are being pursued in recent years.
To manufacture Group-III nitride crystal substrates, vapor-phase techniques enable crystal-growth rates of 100 μm/hr or greater; and among vapor-phase techniques, hydride vapor-phase epitaxy (HVPE hereinafter) is being utilized as powerful tool for rapid crystal growth. (Cf., for example, Isamu Akasaki, Group-III Nitride Semiconductors, 1st Ed., Baifukan, Tokyo, 1999, pp. 67-91.)
Meanwhile, metalorganic chemical vapor deposition (MOCVD hereinafter), in which the crystal growth rate is highly controllable and which is suited to depositing thin films with a planar surface, is being utilized as a powerful tool to grow a Group-III nitride crystal layer onto a base substrate. (Cf. ibid.) A crystal layer thus having been created on a base substrate is usually referred to as a template. For instance, templates that are a layer of Group-III nitride crystal having been grown by MOCVD onto a sapphire plate as a base substrate are being employed as a substitute for Group-III nitride crystal substrates.
Given that a base substrate is necessary, however, to grow Group-III nitride crystal by either the HVPE or MOCVD technique, the dislocation density of a Group-III nitride crystal substrate, or of the Group-III nitride crystal layer on a template, will depend on the dislocation density of its base substrate. Moreover, when Group-III nitride crystal has been grown on a foreign substrate, such as a sapphire or SiC plate, as the base substrate rather than a native Group-III nitride substrate, the dislocation density of the epitaxial Group-III nitride crystal has tended to increase further, owing to the crystal-lattice mismatch, and the difference in thermal expansion coefficient, between the base substrate and the epitaxial crystal layer.
Although HVPE and MOCVD each has its advantages, as noted above—with the former technique crystal growth is rapid, and with the latter, the ability to control crystal growth rates is outstanding—in that they do not enable crystal dislocation density to be adequately lowered, the techniques have not proven to be practical methods for the manufacture of Group-III nitride crystal substrates.
In contrast to HVPE and MOCVD, with liquid-phase techniques, which are typified by flux-growth and the high-nitrogen-pressure solution growth techniques, because crystal growth proceeds in a quasi-equilibratory state that is near equilibrium thermodynamically, compared with vapor-phase techniques, which employ highly reactive gases, nucleation during growth is kept under control, generally yielding Group-III nitride crystal of low dislocation density and superior crystallinity. (Cf., for example, Hisanori Yamane, et al., “Preparation of GaN Single Crystals Using a Na Flux,” Chemistry of Materials, Vol. 9, 1997, pp. 413-416; Fumio Kawamura, et al., “Growth of a Large GaN Single Crystal Using the Liquid Phase Epitaxy (LPE) Technique,” Japanese Journal of Applied Physics, Vol. 42, 2003, pp. L4-L6; and Takayuki Inoue, et al., “Growth of Bulk GaN Single Crystals by the Pressure-Controlled Solution Growth Method,” Japanese Journal of Applied Physics, Vol. 39, 2000, pp. 2394-2398.)
Turning to an example, apart from Group-III nitride crystal, of Group III-V crystal through a vapor-phase technique: Growing a layer of GaAs or AlGaAs crystal onto a GaAs substrate by a vapor-phase technique using self-flux produces a planar crystalline layer whose dislocation density is lower than that of the base substrate, because in this case as well, crystal growth proceeds in a quasi-equilibrium state. Given the low dislocation density of the crystal layers, highly durable red light-emitting diodes may be manufactured by forming electrodes and associated components on the crystal after it is made into chips. Such crystal has therefore been widely used to date in such applications as the light-emitting component in remote controls.
Although, as discussed above, lowering of dislocation density by means of liquid-phase epitaxy has been confirmed in Group-III nitride crystals (Ibid., Fumio Kawamura and Takayki Inoue)—as is the case with the other materials just mentioned—owing to nitrogen's high equilibrium vapor pressure, the amount of nitrogen that dissociates into the vapor phase is generally low, resulting in a crystal growth rate that is a sluggish 10 μm/hr. Moreover, notwithstanding the slowness of the crystal growth, to steadily maintain a thermodynamically quasi-stable state under high temperature and pressure is challenging in terms of the equipment as well as the technology; hence, without being able to adequately control the growth rate, a problem with growing Group-III nitride crystals by liquid-phase epitaxy has been poor surface planarity, an example of which is hexagonally shaped projections, appearing in the crystal surface, that are indicative of a wurtzite type of hexagonal crystalline structure. For these reasons, attempts using liquid-phase epitaxy to produce substrates for optical devices such as blue-violet lasers, blue LEDs, and white LEDs, or for electronic devices such as field-effect transistors, require prolonged periods of time for the crystal growth, and moreover, the crystal must go through an operation to planarize the surface by polishing or a like process; the consequently high manufacturing costs have been prohibitive of making the technique practical.